1. Field of the Invention
The present invention relates to an optical element and a method for evaluating the optical element. More particularly, the present invention relates to an optical element adapted to form an image of an object by use of an off-axial, optical element block which is molded in an integral form and which includes an optically acting surface of a rotationally asymmetric, aspherical surface or a curved surface (off-axial curved surface) a normal to which does not agree with a reference axis at an intersecting point of an optical path of a reference ray (the reference axis) passing from the object plane to the image plane, and a method for evaluating the optical element.
2. Related Background Art
The conventional optical devices for forming the object image on the final image plane have been constructed mostly using a coaxial type optical system comprised of optical elements having a common optical axis. Since such coaxial optical systems tend to be long along the optical-axis direction, compactification thereof has often been attempted by bending the optical path by a plane mirror.
On the other hand, it has been found out that a non-coaxial optical system as an off-axial, optical system disclosed in Japanese Patent Application Laid-Open No. 8-292371 was able to contribute to the compactification of the optical system.
In the off-axial optical systems, an example of which is illustrated in FIG. 8, the concept of the reference axis (which corresponds to an axis from an axis 5-1 to an axis 5-6 in this figure) is adopted as a correspondent to the optical axis of the coaxial systems.
This is defined as an optical path of a center ray out of those arriving at the center of the object plane or the image plane (the both of which will be totally called the focal plane herein). The optical systems of this type are called off-axial, optical systems, because they include a surface a normal to which does not agree with the reference axis, but makes a finite angle except for 0, at a point where the reference axis corresponding to the optical axis intersects with the component surface. (In FIG. 8 the surfaces r2 to r6 are such surfaces.)
Japanese Patent Application Laid-Open No. 9-5650 describes the details of methods for setting surface shapes and computing paraxial amounts for the non-coaxial, off-axial, optical system, together with properties of the off-axial, optical system.
Generally speaking, it is common practice to use asymmetric, aspherical surfaces for the off-axial, optical systems from the viewpoint of correction for aberration. The optical systems are often designed using off-axial reflecting surfaces in order to construct the optical systems in compact structure or in order to suppress occurrence of chromatic aberration.
In the case of the optical systems employing the off-axial reflecting surfaces, it is favorable in terms of accuracy and cost to mold the whole system in the form of an integral, off-axial, optical element block (either a block of a solid (internally filled) type or a block of a hollow type).
In practical formation (production) of the off-axial, optical element blocks, surface precision can vary, so as to produce defectives, depending upon molding conditions. This raises the need for evaluation to check the finish of shapes of the component surfaces in the optical element block. In this evaluation of the shapes of the component surfaces, since the surface shapes are the rotationally asymmetric, aspherical surfaces, measurement and evaluation cannot be made by interference measurement employed for rotationally symmetric, spherical surfaces. Then, the finish of surface shapes is judged normally by measuring a profile with a contact every line of cross section of the shape and scanning the cross-sectional lines.
This measurement, however, involves problems that a lot of time is necessary for the measurement and evaluation of each of the surface shapes of the optical element and that the contact often damages the optical surface itself on the block.
A challenge for mass production of the off-axial, optical element blocks was thus to establish a determination method capable of determining whether the finish of the off-axial, optical element blocks is good or poor, i.e., capable of discriminating surface conditions within a short time.
In the case of the off-axial, optical element blocks of the solid type, inspection-evaluation also has to be executed as to dust and bubbles inside the optical element, an index profile, birefringence, etc., as well as the surface precision, but there have been no conventionally known determination and evaluation methods thereof.